After some 2 to 400,000,000
years the slates-to-be were firm shales that lay beneath younger
formations 5 to 6 miles thick.
As the Paleozoic Era came to an end, the continents were squeezed by rocks beneath the oceans.
They pushed the hardened sea muds upwards, shoving and crumpling them into huge, high folds.When the shales were squeezed into sharp folds, pressure became so great that they raised nearly 6 miles of rock above them crumpling the top-most shale into mountains. The intense heat and pressure generated caused many particles to recrystallize and flatten, often at right angles to direction of compression. This gave the rock a "grain" called "slate cleavage" that allows it to be split easily along one plane but not another.
formations 5 to 6 miles thick.
As the Paleozoic Era came to an end, the continents were squeezed by rocks beneath the oceans.
They pushed the hardened sea muds upwards, shoving and crumpling them into huge, high folds.When the shales were squeezed into sharp folds, pressure became so great that they raised nearly 6 miles of rock above them crumpling the top-most shale into mountains. The intense heat and pressure generated caused many particles to recrystallize and flatten, often at right angles to direction of compression. This gave the rock a "grain" called "slate cleavage" that allows it to be split easily along one plane but not another.
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